Hydrocarbon exploration typically involves various geophysical methods to detect the presence of hydrocarbons in the natural void space of the rock (measured as “porosity’) or to map structural features in a formation of interest which are capable of trapping hydrocarbons.
To be mapped geophysically, the formation containing the hydrocarbon must possess a physical property contrast that the geophysical method responds to. For example, the electrical conductivity (c,), or its inverse, resistivity (p), is a physical property that can be measured with electrical or electromagnetic (EM) methods. The resistivity of a rock depends strongly on the resistivity of the pore fluid and even more strongly on the porosity of the rock. Typical brine in sedimentary rock is highly conductive. The presence of brine in bulk rock renders the rock conductive.
Hydrocarbons are electrically non-conductive. Consequently, bulk resistivity of a rock is reduced when hydrocarbons are present. In general, different rocks in a given sedimentary section have different porosities, so even in the absence of hydrocarbons, information about the sedimentary section can be determined.
Resistivity is typically measured with a direct current (DC) source that injects current into the ground or with low frequency time varying fields. Alternatively, one may measure the magnetic fields produced by the induced current. Thus, by measuring the magnitude of the induced current or the secondary magnetic fields arising from these, it is possible to infer the conductivity of the earth formation.
Electromagnetic surveys typically make use of the fact that the complex formation resistivity is typically measured as a function of the frequency of excitation signal. The complex formation resistivity can be defined as ρ=1/σ+jω∈, where σ is the formation conductivity and ∈ is the formation dielectric constant.
However, at present the inversion of electromagnetic (EM) surveys (aka Deep Electromagnetic Prospecting) is limited to mapping the real part of the formation resistivity with the aim of inferring the saturation distribution in the reservoir. EM methods are ideal in geologic situations where rocks of greatly different electrical resistivity are juxtaposed.
However, conventional inversion of the deep electromagnetic (EM) surveys is limited to determining and mapping of the real part of the formation resistivity with the aim of inferring the saturation distribution in the reservoir.
One aim of an embodiment of the present invention is to describe a method to use EM prospecting or borehole complex resistivity data to determine petrophysical information regarding an earth formation.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.